Abstract
One of the most controversial debates in cognitive neuroscience concerns the cortical locus of semantic knowledge and processing in the human brain. Experimental data revealed the existence of various cortical regions relevant for meaning processing, ranging from semantic hubs generally involved in semantic processing to modality-preferential sensorimotor areas involved in the processing of specific conceptual categories. Why and how the brain uses such complex organization for conceptualization can be investigated using biologically constrained neurocomputational models. Here, we improve pre-existing neurocomputational models of semantics by incorporating spiking neurons and a rich connectivity structure between the model ‘areas’ to mimic important features of the underlying neural substrate. Semantic learning and symbol grounding in action and perception were simulated by associative learning between co-activated neuron populations in frontal, temporal and occipital areas. As a result of Hebbian learning of the correlation structure of symbol, perception and action information, distributed cell assembly circuits emerged across various cortices of the network. These semantic circuits showed category-specific topographical distributions, reaching into motor and visual areas for action- and visually-related words, respectively. All types of semantic circuits included large numbers of neurons in multimodal connector hub areas, which is explained by cortical connectivity structure and the resultant convergence of phonological and semantic information on these zones. Importantly, these semantic hub areas exhibited some category-specificity, which was less pronounced than that observed in primary and secondary modality-preferential cortices. The present neurocomputational model integrates seemingly divergent experimental results about conceptualization and explains both semantic hubs and category-specific areas as an emergent process causally determined by two major factors: neuroanatomical connectivity structure and correlated neuronal activation during language learning.
Highlights
The brain mechanisms of meaning processing have been investigated for many years, cognitive neuroscientists have not reached a consensus about the function and the organizational principles of semantic knowledge
Word-meaning acquisition was simulated under the impact of repeated sensorimotor pattern presentations, in the 3 of the 4 sub-systems, FIGURE 2 | Distributions of cell-assemblies (CAs) emerging in the 12 area network during simulation of word learning in the semantic context of visual perception (A)
Network correlates of action-related words extend into lateral motor cortex (M1L, PML, but not V1), semantically grounding words in information about actions
Summary
The brain mechanisms of meaning processing have been investigated for many years, cognitive neuroscientists have not reached a consensus about the function and the organizational principles of semantic knowledge. Studies of patients with lesions in modality-specific regions revealed category-specific semantic deficits (Warrington and Mccarthy, 1983; Damasio et al, 1996; Neininger and Pulvermüller, 2003; Gainotti, 2010; Trumpp et al, 2013; Dreyer et al, 2015) which can not be explained by symbolic systems accounts presuming category-general semantic hubs Likewise, these findings challenge proposals that see the semantic processing role of sensorimotor areas as optional, ancillary or epiphenomenal and deny them a genuine semantic conceptual function (Machery, 2007; Mahon and Caramazza, 2008; Caramazza et al, 2014). The evidence for multiple hubs and modality-specific areas for conceptual-semantic knowledge is difficult to reconcile within most current neurobiological models of symbol processing
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